Clipping 2 and 3 Point

Clipping is a way to cut brushes into whatever shape you want as long as it's a convex.
You can cut one brush at a time, or several.
You can define a cut with two points or three.
Time to show some examples.

Two-Point clipping.
Make a cube-shaped brush in the editor, say 128-128-128. Use grid size 64 (number 7 is the short cut key)
It should look like this:

Notice I used the texture common/caulk. I do this out of habit because a lot of my brushes are turned into detail and this saves me from applying caulk to the unseen faces later.
Onto clipping it. While the new brush is still selected, hit the (X key) this turns on the clipping tool. You could hit the little menu button to turn it on also.
Let's change the grid size to 16 (number 5 is the short cut key).
Go to your xy view (top looking down).
Now left click one grid over to the right from the bottom left.
Like this.

Next left click one grid up from the bottom left.
Like this.

Now we have a clipping plane made that will split the brush between these two points.
Notice that your pointer turns into a little + when you are by one of these points. You can change where clipping points go by left clicking on one of them and dragging it to a new point.
Also note that the bottom left-hand corner has turned red. This is the part that would be deleted if we were to hit the enter key.
When placing clipping points do them in a clockwise fashion. The part to the left will always be the red part.
Don't worry if you mess up because you can always hit ctrl+enter and this will change what part of the brush is red.
Now if you wanted to keep both sides of a clipped brush hit shift+enter.
That's it for 2 point clipping. Go ahead cut up a few brushes. Do it in all the 2d views and try different grid sizes as well.

Note don't use a grid size smaller than 2 as you will get some bad cuts.

Try cutting several brushes at the same time as in this example.

Notice I hit shift+enter to save both sides of the clip. Now it's time to get rid of the scrap brushes. In the 3d view, left click on the large brushes to unselect them all. The only ones that should still be selected are the little triangle brushes in the corner. Now hit the backspace key to delete them. 

Three Point clipping
Don't be afraid of three point clipping it does come in handy.
Now let's begin with the same brush as in the beginning of this tutorial. With the two points already there, switch to a side view (ctrl+tab) use the xz front view. Now place the third clipping point on the bottom left.

Notice how the brush is cut in a way that couldn't be done with only 2 points. Try moving the third point around watch how it looks in the 3d view. Also try hitting ctrl+enter to change what part of the brush would be deleted when you hit enter.
Before you hit enter to delete the red part check all views to make sure your cutting points are where you want them and also check your 2d views to confirm what part gets deleted. As the 3d view can fool ya I know this is a bug. I'm using Q3Radiant 202. Anyway double check first, then hit enter.
Make the same cuts in all the corners checking all views before hitting enter. When you are done the brush should look like this.

By the time you get this brush done, you will have a basic understanding of how three point clipping works.
Only time and many brushes later will you have it mastered as you have to think of cutting points along angles that are only viewed in only three separate planes (top view and both side views). It's a good thing to use the 3d view to confirm your cut.

Shortcut keys that come in handy for this.
X KEY = turns on the clipping tool
ctrl+enter with clipping tool on already on = to change what part of the brush is clipped.
shift+enter with clipping tool already on = keep both brushes that are formed by the clip.
shift left click = to select multiple brushes